It is known that the performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft powered chain drive or belt drive. It is also known that engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft. Heretofore, such changes in engine valve timing have been accomplished by a separate hydraulic motor operated by engine lubricating oil. However, this actuating arrangement consumes significant additional energy and it increases the required size of the engine lubricating pump because of the required rapid response time for proper operation of the camshaft phasing actuator. Further, these arrangements are typically limited to a total of 20.degree. of phase adjustment between crankshaft position and camshaft position, and typically such arrangements are two-position arrangements, that is, on, or fully phase adjusted as one position, or off, or no phase adjustment, as a second position. The present invention is designed to overcome these problems associated with prior art variable camshaft timing arrangements by providing a self-actuating, variable camshaft timing arrangement which does not require external energy for the operation thereof, which does not add to the required size of the engine lubricating pump to meet transient hydraulic operation requirements of such variable camshaft timing arrangement, which provides for continuously variable camshaft to crankshaft phase relationship within its operating limits, and which provides substantially more than 20.degree. of phase adjustment between the crankshaft position and the camshaft position. Prior U.S. Patents which describe various systems of the foregoing type are U.S. Pat. Nos. 5,046,460, 5,002,023, and 5,107,804, the disclosures of each of which are hereby incorporated by reference.
Inventions disclosed in the prior art provide a method for phase adjustment of an internal combustion engine in which the position of the camshaft, or the positions of one or both of the camshafts in a dual camshaft system, is phase adjusted relative to the crankshaft by an actuating arrangement. Such an arrangement is controlled by a robust closed loop system having a hydraulic pilot stage with a pulse width modulated (PWM) solenoid, for example, a system such as generally disclosed by co-pending U.S. patent application Ser. No. 07/847,577, which is hereby incorporated by reference. A predetermined set point dictates the desired camshaft phase angle for certain engine performance criteria. This variable camshaft timing (VCT) system can be used to improve important engine operating characteristics such as idle quality, fuel economy, emissions or torque. A preferred embodiment of a camshaft mounted hydraulic VCT mechanism uses one or more radially extending vanes which are circumferentially fixed relative to the camshaft and which are receivable in cavities of a sprocket housing that is oscillatable on the camshaft. Hydraulic fluid is selectively pumped through a proportional (spool) valve to one side or another of each vane to advance or retard the position of the camshaft relative to the sprocket. A pumping action occurs in reaction to a signal generated by a closed loop feedback system. Closed loop feedback control is imperative for any but the "two-position" case, i.e., fully advanced or fully retarded. This is because camshaft phase is controlled by the integral of the spool valve position. That is, spool position corresponds not to camshaft phase, but to its rate of change. Thus, any steady state spool position other than null (centered) will cause the VCT to eventually go to one of its physical limits in phase. Closed loop control allows the spool to be returned to null as the camshaft phase reaches its commanded position or set point. An additional result of using feedback control is that the system performance is desensitized to mechanical and environmental variations. This results in a reduction of the effects of short term changes, such as changes in oil pressure or temperature, or long term variations due to tolerances or wear. In addition, set point tracking error in the presence of unanticipated disturbances (e.g. torque shifts) is reduced. A degree of sensitivity reduction and disturbance rejection is referred to as the "robustness" of the control system. Closed loop control can thus provide stable set point tracking with some degree of robustness.